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Heat exchanger fouling by precipitation of calcium phosphates Singh, Atmajeet
Abstract
The fouling characteristics of artificially hardened waters containing calcium and orthophosphate ions were investigated. The water was circulated through a steam heated double pipe heat exchanger under turbulent flow conditions. Inlet water temperatures were about 20 to 25°C and the phosphate content about 100 ppm. Experiments were carried out to ascertain the effects of pH, velocity, surface temperature and concentrations of the calcium and the orthophosphate on the fouling process. The fouling behaviour was recorded in terms of the fouling resistance-time curve which was calculated from the time dependence of the overall heat transfer coefficients. Under the constant heat flux condition imposed, the fouling resistance was generally linear in time; however, asymptotic fouling was observed in some cases. The fouling rate was found to be a very strong function of the pH over the range 6.5 to 8.0. The rate was also found to be exponentially dependent on the calculated surface temperature. The initial fouling rate increased linearly with the velocity and with the concentrations of scale forming species. Deposit thickness increased with heated length of the stainless steel test section. In most cases, the deposit was a dense crystalline scale, which had a Ca/P molar ratio of 1.35±0.15. The results are discussed in terms of the ionic equilibrium involved and the solubility and crystallisation kinetics from the literature. It is apparent that the rate of phosphate precipitation is influenced both by surface reaction and mass transfer in the range of the variables studied. Findings are compared with the few prior phosphate fouling studies from the literature. Scale density is reported, and its thermal conductivity estimated using the fouling resistance measurements.
Item Metadata
Title |
Heat exchanger fouling by precipitation of calcium phosphates
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Creator | |
Publisher |
University of British Columbia
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Date Issued |
1992
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Description |
The fouling characteristics of artificially hardened waters containing calcium and
orthophosphate ions were investigated. The water was circulated through a steam heated double
pipe heat exchanger under turbulent flow conditions. Inlet water temperatures were about 20 to
25°C and the phosphate content about 100 ppm. Experiments were carried out to ascertain the
effects of pH, velocity, surface temperature and concentrations of the calcium and the
orthophosphate on the fouling process. The fouling behaviour was recorded in terms of the
fouling resistance-time curve which was calculated from the time dependence of the overall heat
transfer coefficients. Under the constant heat flux condition imposed, the fouling resistance was
generally linear in time; however, asymptotic fouling was observed in some cases.
The fouling rate was found to be a very strong function of the pH over the range 6.5 to
8.0. The rate was also found to be exponentially dependent on the calculated surface
temperature. The initial fouling rate increased linearly with the velocity and with the
concentrations of scale forming species. Deposit thickness increased with heated length of the
stainless steel test section. In most cases, the deposit was a dense crystalline scale, which had
a Ca/P molar ratio of 1.35±0.15. The results are discussed in terms of the ionic equilibrium
involved and the solubility and crystallisation kinetics from the literature. It is apparent that the
rate of phosphate precipitation is influenced both by surface reaction and mass transfer in the
range of the variables studied. Findings are compared with the few prior phosphate fouling
studies from the literature. Scale density is reported, and its thermal conductivity estimated using
the fouling resistance measurements.
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Extent |
6007242 bytes
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Genre | |
Type | |
File Format |
application/pdf
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Language |
eng
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Date Available |
2008-12-17
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Provider |
Vancouver : University of British Columbia Library
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Rights |
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.
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DOI |
10.14288/1.0058511
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URI | |
Degree | |
Program | |
Affiliation | |
Degree Grantor |
University of British Columbia
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Graduation Date |
1992-11
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Campus | |
Scholarly Level |
Graduate
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Aggregated Source Repository |
DSpace
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Item Media
Item Citations and Data
Rights
For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.